Actual Earthquake Motion Usually Overestimated

In an earlier column I explained that the true ground motion 60 miles distant from a magnitude 8 earthquake would total only about an inch and a half of actual back-and-forth shaking. This statement spawned comments by several readers to the general effect that either: (a) there was a misprint in the article, or (b) I should enter some other line of work.

Having been through a number of fairly large earthquakes myself, I can sympathize with the feeling that the ground must be moving more than an inch or so when you can barely stand. What is it, then, that translates these relatively minor movements into such apparently gargantuan ones?

There are several reasons. One of the most important is that most instruments measure seismic waves that have traveled from the source to a seismograph located on solid bedrock. However, few structures actually have their foundations on bedrock, but are situated instead on tens or hundreds of feet of overburden. The overburden is much more pliant and free to shake than is the bedrock, and this sets up larger oscillations at the surface.

In addition, all structures have one or more natural frequencies of oscillation. This means that, if Superman were to give the corner of a building a push, it would tend to rock back and forth at a particular frequency for a while, just like a car with worn-out shock absorbers. If the frequency of earthquake shaking is close to the natural resonant frequency of the structure or one of its harmonics, the resulting amplitude rapidly grows. The longer the shaking continues, the more serious the consequences become.

It is natural to think of the seismic wave amplitude (the actual distance that the ground moves back and forth) as being the main cause of earthquake damage. Actually, it is acceleration (the abrupt changes to and fro) that does the most harm. To illustrate this, picture two men holding a plank on which another is standing. If they walk slowly to another corner of a room, the man standing on the plank can probably maintain his balance. Although the amplitude of the motion is large, the acceleration is small. But now imagine what would happen if the men holding the plank were to jerk it sharply back and forth between them. The displacement is small, but the acceleration is large and the guy on top will probably fall off. This is why it is difficult to stand during large earthquakes, and why people often estimate ground motion to be larger than it actually is.

Sometimes the senses do play tricks during the confusion and disorientation of an earthquake, but often the most improbable tales are true.

For example, a man in Valdez was watching his son some distance away when the 1964 magnitude 8.4 earthquake struck. He later recalled that the ground rose up and dropped between them so that at one moment, he could see only his son's head, and the next, he could see his entire body. Unlikely though this may sound, subsequent investigation pointed out that this was probably an accurate account--that waves known as "ground roll" were passing between the two, placing one of them on a crest and the other in a trough as the waves went by.

Another example deals with the experience of a well-known and respected seismologist in California many years ago. The scientist was in his seismographic vault working on some equipment when an earthquake occurred. As it was going on, he claims to have observed very small waves--a few inches long and fractions of an inch high--traveling across the concrete floor of the vault. So convinced was he that he had seen them that he afterward inspected the floor with a magnifying glass to see if he could find cracks that they might have caused. Of course, seismic waves travel much too fast to be observed moving across a floor, and the formation of waves with the resultant flexing that they would cause in the thick concrete would be an impossibility in the reinforced floor of a sturdy seismographic vault.

Although the seismologist knew that what he thought he had seen was impossible, the memory always remained in his mind's eye, just as he saw's it on that day.